Influence of carbon source on cell surface topology of Thermomonospora curvata

Effects of the detergent Tween 80 on Thermomonospora curvata

Tween 80 (0.1%, v/v) added to Thermomonospora curvata growing in minimal medium caused a transient lowering of the dry cell mass, decreased the optimal growth temperature of the thermophile from 62 to 54°C, and increased extracellular esterase activity. Cells grown in the presence of Tween 80 had decreased concentrations of branched chain fatty acids and increased concentrations of oleic acid. The detergent removed surface protuberances from mycelia and increased the liberation of enzymes active against crystalline cellulose, but did not stimulate liberation of enzymes active against carboxymethylcellulose, starch or pectin.

Corn fiber hydrolysis by Thermobifida fusca extracellular enzymes

Thermobifida fusca was grown on cellulose (Solka-Floc), xylan or corn fiber and the supernatant extracellular enzymes were concentrated. SDS gels showed markedly different protein patterns for the three different carbon sources. Activity assays on a variety of synthetic and natural substrates showed major differences in the concentrated extracellular enzyme activities. These crude enzyme preparations were used to hydrolyze corn fiber, a low-value biomass byproduct of the wet milling of corn. Approximately 180 mg of reducing sugar were produced per gram of untreated corn fiber. When corn fiber was pretreated with alkaline hydrogen peroxide, up to 429 mg of reducing sugars were released per gram of corn fiber. Saccharification was enhanced by the addition of beta-glucosidase or by the addition of a crude xylanase preparation from Aureobasidium sp.

Surface structures of new and lesser known species of thermobifida as revealed by scanning electron microscopy

Surface structures of representatives of the genus Thermobifida were examined by scanning electron microscopy. Spores formed at the tips of multibranched sporophores initially resembled short sausages; then, upon maturation, they gradually built up their typical ovoid shape. Characteristic differences were observed between T. cellulolytica strain TB108 and T. fusca strains TM51. The spores of TB108 were larger (0.8x 1.3 microm) than those of TM51 (0.6 x 1.1 microm) in consequence of the more thickened outer squamous layer. When Thermobifida strains were grown on cellulose as sole carbon source, the mycelium was found to coil around the cellulose crystals and multiple protuberances emerged, resulting in a scabrous appearance to the mycelial surface. The presence of these cellulosome-like structures yielded a 24.5% surface enlargement of the scabrous mycelium as compared with the smooth one. The cellulosome emergence pattern paralleled the proportional increase in free endoglucanase activity measured during the culturing of these actinomycetes in the presence of cellulose.

Regulation of cellulose-inducible structures of Clostridium cellulovorans

Scanning electron microscopy was used to detect ultrastructural protuberances on the cellulolytic anaerobe Clostridium cellulovorans. Numerous ultrastructural protuberances were observed on cellulose-grown cells, but few were detected on glucose-, fructose-, cellobiose-, or carboxymethylcellulose (CMC)-grown cells. Formation of these protuberances was detected within 2 h of incubation in cellulose medium, but 4 h incubation was required before numerous structures were observed on the cells. When a soluble carbohydrate or CMC was mixed with cellulose-grown cells, the ultrastructural protuberances could no longer be detected. In fact, no protuberances were observed within 5 min following the addition of glucose, cellobiose, or methylglucose to cellulose-grown cells. The presence of these protuberances corresponded with the binding of the Bandeiraea simplicifolia BSI-B4 isolectin to the cell. Cellulose-grown cells had a greater level of observable lectin binding than cellobiose-grown cells, and lectin binding was not detected on glucose- or fructose-grown cells. In addition, lectin binding ability was lost by cellulose-grown cells following the addition of glucose, fructose, or methylglucose to the cellulose medium. A cellulose-affinity protein fraction expressing cellulase activity was also detected in cell extracts of cellobiose- or cellulose-grown cultures. However, this protein fraction was not detected in extracts of glucose-grown cultures, and was rapidly lost (within 5 min) following the addition of glucose to cellulose-grown cultures. The ability of C. cellulovorans to adhere to cellulose was also affected by the energy substrate, but not in the same manner as the protuberance formation or the cellulase-containing protein fraction. Rather, cellobiose-, cellulose-, and CMC-grown cultures adhered to cellulose, but this adherence was not affected by addition of glucose to the medium. This is the first report that soluble carbohydrates caused the rapid loss of some cellulose-inducible systems of C. cellulovorans.

Cationized ferritin as a stain for electron microscopic observation of bacterial ultrastructure

The electron microscope has proven very effective for visualization of various morphological features of bacteria. Cationized ferritin (CF) is a stain commonly used to increase the electron microscopic resolution of bacterial cells, thereby enabling detailed analysis of their morphological and structural features. CF has been useful for microscopic examination of the bacterial capsule, cell wall, S-layer, and various unique morphological structures. In addition, as a cation, CF binds only to negatively charged molecules. Thus, CF has been used to identify sites of anionic charge on the bacterial cell surface, which has led to insights concerning the formation and turnover of bacterial peptidoglycan and the S-layer proteins. As a cation, however, CF may also interact with certain cellular components, causing erroneous interpretation of microscopic results. This review provides a discussion of both the strengths and weaknesses of CF when used as a stain for electron microscopy.

Comparison of staining techniques for scanning electron microscopic detection of ultrastructural protuberances on cellulolytic bacteria

Cell surface protuberances found on cellulolytic bacteria, but not on noncellulolytic bacteria, can be detected by scanning electron microscopy. Cationized ferritin typically has been used as a stain to increase the microscopic resolution of these protuberances; however, as a cation it binds only to negatively charged molecules. Thus, binding of cationized ferritin to cell surface molecules can be affected by the cell's physiological state. We incubated the noncellulolytic bacterium, Clostridium beijerinckii, in different media at various temperatures to obtain cells with different growth rates and physiological states. Staining of these cells with cationized ferritin showed that slower growing cells exhibited more protuberant structures than faster growing cells. This provided a clear correlation of ultrastructural protuberances with physiological changes associated with growth rate. On the other hand, cells stained with osmium tetroxide exhibited no protuberant structures regardless of growth rate. Because various cations are known to induce aggregation of surface proteins on some cellulolytic Clostridia, we incubated the cellulolytic bacterium, Clostridium cellulovorans, in media containing glucose, cellobiose, or cellulose. Ultrastructural protuberances were evident on all cells stained with cationized ferritin, but extensive protuberances were detected only on cells grown in cellulose and stained with osmium tetroxide. For cells stained with cationized ferritin, the presence of ultrastructural protuberances was correlated with growth rate rather than induction of cellulolytic systems. By contrast, cells stained with osmium tetroxide showed a clear correlation between protuberant structures and cellulolytic activity.

OlpB, a new outer layer protein of Clostridium thermocellum, and binding of its S-layer-like domains to components of the cell envelope

Several proteins of Clostridium thermocellum possess a C-terminal triplicated sequence related to bacterial cell surface proteins. This sequence was named the SLH domain (for S-layer homology), and it was proposed that it might serve to anchor proteins to the cell surface (A. Lupas, H. Engelhardt, J. Peters, U. Santarius, S. Volker, and W. Baumeister, J. Bacteriol. 176:1224-1233, 1994). This hypothesis was investigated by using the SLH-containing protein ORF1p from C. thermocellum as a model. Subcellular fractionation, immunoblotting, and electron microscopy of immunocytochemically labeled cells indicated that ORF1p was located on the surface of C. thermocellum. To detect C. thermocellum components interacting with the SLH domains of ORF1p, a probe was constructed by grafting these domains on the C terminus of the MalE protein of Escherichia coli. The SLH domains conferred on the chimeric protein (MalE-ORF1p-C) the ability to bind noncovalently to the peptidoglycan of C. thermocellum. In addition, 125I-labeled MalE-ORF1p-C was shown to bind to SLH-bearing proteins transferred onto nitrocellulose, and to a 26- to 28-kDa component of the cell envelope. These results agree with the hypothesis that SLH domains contribute to the binding of exocellular proteins to the cell surface of bacteria. The gene carrying ORF1 and its product, ORF1p, are renamed olpB and OlpB (for outer layer protein B), respectively.

The biological degradation of cellulose

Cellulolytic microorganisms play an important role in the biosphere by recycling cellulose, the most abundant carbohydrate produced by plants. Cellulose is a simple polymer, but it forms insoluble, crystalline microfibrils, which are highly resistant to enzymatic hydrolysis. All organisms known to degrade cellulose efficiently produce a battery of enzymes with different specificities, which act together in synergism. The study of cellulolytic enzymes at the molecular level has revealed some of the features that contribute to their activity. In spite of a considerable diversity, sequence comparisons show that the catalytic cores of cellulases belong to a restricted number of families. Within each family, available data suggest that the various enzymes share a common folding pattern, the same catalytic residues, and the same reaction mechanism, i.e. either single substitution with inversion of configuration or double substitution resulting in retention of the beta-configuration at the anomeric carbon. An increasing number of three-dimensional structures is becoming available for cellulases and xylanases belonging to different families, which will provide paradigms for molecular modeling of related enzymes. In addition to catalytic domains, many cellulolytic enzymes contain domains not involved in catalysis, but participating in substrate binding, multi-enzyme complex formation, or possibly attachment to the cell surface. Presumably, these domains assist in the degradation of crystalline cellulose by preventing the enzymes from being washed off from the surface of the substrate, by focusing hydrolysis on restricted areas in which the substrate is synergistically destabilized by multiple cutting events, and by facilitating recovery of the soluble degradation products by the cellulolytic organism. In most cellulolytic organisms, cellulase synthesis is repressed in the presence of easily metabolized, soluble carbon sources and induced in the presence of cellulose. Induction of cellulases appears to be effected by soluble products generated from cellulose by cellulolytic enzymes synthesized constitutively at a low level. These products are presumably converted into true inducers by transglycosylation reactions. Several applications of cellulases or hemicellulases are being developed for textile, food, and paper pulp processing. These applications are based on the modification of cellulose and hemicellulose by partial hydrolysis. Total hydrolysis of cellulose into glucose, which could be fermented into ethanol, isopropanol or butanol, is not yet economically feasible. However, the need to reduce emissions of greenhouse gases provides an added incentive for the development of processes generating fuels from cellulose, a major renewable carbon source.